1. Field of the Invention
The present invention relates to an image forming optical device for forming an optical image of an object on a surface of photoelectric converter in an image sensing element and an image sensing device for incorporating such the image forming optical device and the image sensing element, preferable for use in video camera or digital still camera associated with personal computer (PC) or mobile terminal unit (such as a cellular phone, or a mobile PC), particularly, relates to an image sensing device of which total length can be shortened in comparison with a conventional optical lens system, wherein the image sensing device is equipped with a focus controlling function without extending the total length.
2. Description of the Related Art
An image sensing device is used in video camera or digital still camera for capturing an optical image and converting the optical image into a video signal. Recently, such the image sensing device is also used in a mobile terminal unit (such as a cellular phone, a mobile PC), a videophone system or a video camera for security system.
The image sensing device includes an image forming optical device and a CCD (charge-coupled device). The image forming optical device has a plurality of optical lenses to capture and focus an optical image to pass it to a CCD. The CCD converts the optical image into a video signal to provide the same to various signal processors.
Recently, a mobile terminal unit becomes a multifunctional unit that can transmit not only an audio information but also a visual information such as video pictures and still pictures. As a natural result, a camera portion is utilized for the mobile terminal unit. However, the conventional camera portion is not adequate to be incorporated in the mobile terminal unit because of its size. An optical device in the camera portion can be shortened or miniaturized by reducing a number of lenses but simply reducing the number of lenses may affect the performance of capturing the optical image.
Therefore, there needed an image forming optical device and an image sensing device in a smaller and compact size, preferable in use for a mobile terminal unit, PC, security system and other similar systems.
FIG. 1 is a cross-sectional view of an image forming optical device having a prism lens. In FIG. 1, the image forming optical device is composed of an aperture 1a for limiting the amount of light, a prism lens 1b for passing an optical image to a CCD not shown.
The prism lens 1b is made of high transmittance material such as optical glass or optical plastic. The prism lens 1b has a first interface 1b1 to pass the optical image, and second interface 1b2 and third interface 1b3 to reflect the image inside the prism lens 1b respectively, and a forth interface 1b4 to pass the optical image along the optical axis 100. The length L is a total length of the optical device where the optical image inputs and outputs.
The first and second interfaces 1b1, 1b2 have a concave shape respectively, and the third and forth interfaces 1b3 , 1b4 have a convex shape respectively.
The optical image refracts diffusely through the first interface 1b1, and proceeds inside the prism lens 1b, and reflects by the second interface 1b2. The reflected image from the second interface 1b2 then proceeds inside the prism lens 1b and reflects by the third interface 1b3. The optical image reflected from the third interface 1b3 converges at the point out side of the prism lens 1b to form an optical image 1c on the CCD not shown.
The prism lens 1b can inflect the optical path along the optical axis 100 so that the length L can be shortened without losing the performance of the optical device. Consequently, the device can be utilized in a small and compact mobile terminal unit.
However, it is difficult to form the prism lens 1b in a predetermined shape, as it requires precise work on processing the first through forth interfaces of the prism lens 1b. 